Multiphase systems, that is, systems that comprise more than one phase, are often used in several industrial processes. In those cases in which gas is used, often this is bubbled throughout a fluid containing one of more disperse phases or wherein said liquid is in a solution. In these systems, the number, velocity and size of gas bubbles play a key role in the process performance. Particularly the total volume fraction taken by the gas or the gas volume fraction is a variable that includes the previously mentioned properties of bubbles and, therefore, its measurement is of critical importance for the characterization of the gas dispersion.
An example of multiphase systems is the ore flotation in which three phases take part: solid, liquid and gas. This process is used in the ore processing industry for the separation of valuable minerals from those without an economic value, while in the energy flotation industry said process is used to separate bitumen drops from sand particles. Among other applications related to the environmental protection are the removal of ink particles from paper fibers in the pulp and paper industry and the wastewater treatment or effluent treatment.
The ore flotation is a separation process, which is based on the differences among the surface properties of particles. The process consists of the gas dispersion, generally air, to form bubbles inside a tank containing a suspension of ore particles in water, thereby forming an ore pulp. This pulp is conditioned with chemical reagents such as collectors, depressants and activators, which purpose is modifying the particle surface properties. Collectors, particularly, function to create or increase the hydrophobicity of the surface exposed to the ore that is aimed to be recovered, that is, preventing the surface from hydrating. This process is assisted by the pH control of the pulp by dosing the pH modifiers such as lime.
Another chemical reagent is the frother (s), which is added to the pulp, they are then, absorbed on the bubbles surface, thereby delaying coalescence among particles, that is, the reduction of the surface area of bubbles due to the formation of particles having a greater volume originating from the smaller ones. In addition, frothers facilitate and stabilize the formation of a froth layer on an upper part of the tank when increasing the water fraction being dragged by the bubbles towards the froth.
The bubbles when freely ascending from the lower part of the tank (flotation machine) through the conditioned pulp, collide with particles forming particle/bubble aggregates in those cases wherein the surface of said particles is hydrophobic, which continue ascending until leaving the collection zone and entering into the froth zone. The hydrodynamic conditions must promote the homogenous distribution of particles and bubbles, and the formation of stable bubble/particle aggregates to minimize the detachment of valuable mineral particles attached to the bubble surface while they are ascending. In this way, the valuable ore-enriched froth overflows by an upper edge of the flotation machine, thereby producing a flow of the product called concentrate. Generally, the hydrophilic particles (gangue) do not attach to the bubbles and they are reported in the other product called tailings. The metallurgical performance of the flotation process and, in turn, financial revenues of the concentrator plant are determined by the valuable ore recovery (quantity) as well as the obtained concentrate ore grade (quality).
Due to the magnitude of the mining operations, which translates into a high treatment capacity, a small improvement in the metallurgical flotation process performance would result in a significant economic benefit, in addition to the environmental benefits regarding the hydric resource management. Considering that the ore grade is rapidly decreasing, it becomes imperative the optimization of the flotation process, to maintain the market competitiveness.
The optimization of the flotation process operation requires, among other initiatives, developing new sensors installed in line, which allow obtaining relevant data regarding the process status for the real-time decision-making. Currently, the in-line sensors in flotation machines are scarce and they are mainly limited to measuring airflows, pulp pH, and froth heights and in some cases, they measure ore grade by using X-ray fluorescence analyzers. Recently, image processing systems have been implemented which by means of the use of digital cameras installed on the flotation machines, monitor the froth surface and determine parameters such as: overflow velocity, color and texture of the froth. In this way, operators only react towards the changes they can observe on the froth surface, from which it is difficult to infer the current process status due to the multivariable and interactive nature of the phenomena that take place in the flotation process. Therefore, a problem faced by circuit flotation operators to find and control more efficient operating conditions to maximize the process metallurgical performance is the lack of instruments for characterizing both the pulp and the gas dispersion in line and in real time.
Studies have shown that variables characterizing the gas dispersion in the form of bubbles, such as the gas surface flow, the gas volume fraction and the bubble size play a key role in the determination of the metallurgical flotation process performance. Particularly, the gas volume fraction in the collection zone has been found to be directly correlated to the bubble surface area flux as described in the document titled “Gas dispersion properties: bubble surface area flux and gas holdup” (2000) by Finch, Xiao, Hardie and Gomez and the document titled “Gas dispersion in column flotation and its effect on recovery and grade” (2012) by Lopez-Saucedo, Uribe-Salas, Pérez-Garibay and Magallanes-Hernández. On its part, the bubble surface area flux is correlated to the particle collection kinetics and, therefore, to the recovery, as described in the article titled “Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell. Part 5: Validation of Sb relationship and effect of froth depth” (1998), by Gorain, Napier-Moon, Franzidis and Manlapig which is included as reference. On the other hand, it has been found in laboratory columns working with water and gas, that the gas volume fraction in the gas collection zone partially determines the water flow reported in the concentrate as described in the document titled “A frother-related research at McGill University” (2006) by Finch, Gelinas and Moyo, which is related to the gangue fine particle dragging and therefore to the grade ore reduction. Recently, a study carried out on industrial flotation columns in different zinc concentrator plants has confirmed a direct relationship between the gas volume fraction at the collection zone and the zinc recovery and an inverse relationship with the enrichment ratio, that is, the zinc ore grade in the concentrate divided by the zinc ore grade in the feed, as described in the document titled “Gas dispersion in column flotation and its effect on recovery and grade” (2012), by Lopez-Saucedo, Uribe-Salas, Pérez-Garibay and Magallanes-Hernández.
The previously described studies reveal the important information contained in the gas volume fraction regarding the metallurgical flotation process performance and suggest its use for optimizing the process.
The present invention has as a purpose reducing the lack of instruments in the flotation process by means of the development of a sensor allowing the in-line and real time measurement of the gas volume fraction on an aerated ore pulp inside the flotation machines for the control and optimization of the metallurgical process performance.